This invention relates to a process for kiln drying wood. In known processes, air is circulated through the stacked wood in a kiln chamber. The temperature and humidity of the air in the kiln are controlled in accordance with established kiln schedules which have been developed for various sizes, conditions and types of wood. A typical schedule, taken from the U.S. Department of Agriculture Handbook #133, DRY KILN OPERATOR'S MANUAL, for Eastern White Pine is shown below:
______________________________________ Moisture content Dry Bulb Air Wet Bulb Air of wood at start Temperature Temperature of step (in %) (in .degree. F.) (in .degree. F.) ______________________________________ Above 60% 130 115 60 130 110 50 130 105 40 130 100 35 130 95 30 140 105 25 150 115 20 160 125 15 160 can vary ______________________________________
Schedules such as the one shown above when closely followed can result in good quality wood being dried in a reasonably fast time with little or no damage. The schedule shown above would take about one week to complete. The temperature is normally expressed in .degree.F. or .degree.C. and the moisture content of the air is usually expressed in .degree.F. or .degree.C. on a wet bulb thermometer. The moisture content may also be expressed in some other manner, such as relative humidity, dewpoint, moisture ratio, etc.
There are several known processes for controlling the temperature and humidity of the air in the kiln. This invention relates to the dehumidification process. The prior art dehumidification systems do not have the ability to operate over a wide range of temperature as shown in the typical schedule above. This invention allows a dehumidification system to operate at any temperature between aproximately 70.degree. F. dry bulb and 160.degree. F. dry bulb. The prior art dehumidifiers have been generally limited to a maximum operating temperature of approximately 120.degree. F. If the temperature is limited to below 120.degree. F., the drying process is much slower, the possibility of damage from mold and stain increases, and kiln operators are required to ignore the established kiln schedules. This invention allows the existing kiln schedules to be used with only minor modifications. It also allows a faster drying time because of the higher temperatures. Problems related to mold are also reduced.
A dehumidification system uses a conventional refrigeration cycle. In the known systems, air is drawn from the kiln chamber and it passes over a cooling coil. It is cooled and dehumidified and then the air passes over a heating coil where it is reheated. A fan is used to draw the air over these coils and then the air is returned to the kiln, heated and with the moisture removed. The cooling coil is an evaporator of a conventional refrigeration cycle. The heating coil is the condenser of a conventional refrigeration cycle and in the refrigeration system there is also a compressor.
In the prior art, the cooling coil, condenser coil and compressor are selected to operate within a certain range of temperatures. If the temperature increases beyond design selection range, the pressures that the compressor is required to maintain in the evaporator and in the condenser also increase and the resulting load would be beyond the design range for the compressor motor. Also, as the temperature of the air going across the cooling coil increases, the refrigerant leaving the cooling coil also increases in temperature. Since the refrigerant cools the compressor in most dehumidification systems, the warmer refrigerant may be unable to provide the cooling the compressor motor requires.
This invention has for its purpose to provide for operating over a wide range of temperatures, to increase the rate of drying thereby to reduce damage from mold and stain, to allow existing kiln schedules to be used with only minor modification and to vary the amount of air passing over the cooling unit of the dehumidifier in such a way as to prevent the compressor from being overloaded and from being overheated.